What has not been stated enough:
In January, Mamdani started with 6% and next to zero name recognition.
Voters are starved for candidates getting personal, approachable and laser-focused on identifying and talking about their everyday problems, not ideology, not the other Party. It is absolutely possible to get from 0 to over 50% in a very short amount of time and without massive ads and donations if you focusing on pleasing voters, not donors.
Next, turnout is everything: if more people vote, Dems win. This could be devastating in all the gerrymandered districts that have been optimized to go to Republicans with a narrow margin.
and third, Young Men by a large margin voted for an Immigrant Socialist strong on Women's and Trans' Rights, proving that it is not xenophobia and misogyny that was powering support among this group for Trump, but the strong desire for genuine change.
Dems have all the ingredients ready for a 2/3 takeover in the Midterms - they just have to want to win more than wanting to please their donors and Republican colleagues.